MEV & mempool FAQ
Plain answers to the questions people ask before turning on MEV protection.
What is the mempool?
The mempool (“memory pool”) is the public waiting room for transactions: after you broadcast a transaction it sits here, visible to everyone, until a validator includes it in a block. Because it’s public, bots can see your pending trade and try to profit from it.
What is MEV?
MEV (Maximal Extractable Value) is the profit bots and block builders can make by reordering, inserting or censoring transactions in a block. The most harmful kind for everyday users is the sandwich attack, where a bot trades just before and just after your swap to skim value from your price.
What is a sandwich attack?
A bot spots your pending swap in the public mempool, buys the same asset right before you (pushing the price up), lets your trade execute at the worse price, then sells right after — pocketing the difference. MEV-protection services stop this by keeping your transaction out of the public mempool.
Public vs private mempool — what’s the difference?
In the public mempool your transaction is visible to every bot before it’s mined. A private mempool (or private RPC) sends your transaction straight to block builders without broadcasting it publicly, so there’s nothing for sandwich bots to see. Some services go further and auction the harmless “back-run” value back to you.
Are MEV-protection RPCs free?
Yes — the major ones (MEV Blocker, Flashbots Protect, MetaMask Smart Transactions) are free to use. They fund themselves from harmless back-run value, and several share a large chunk of it back with you.
Do I lose anything by using a private RPC?
There are trade-offs. Private routing can add a small inclusion delay, and you’re trusting that service’s builder set with your order flow, which is more centralized than the open mempool. For most users the protection is well worth it; for very latency-sensitive or trust-minimized needs, weigh the trade-off (or use an encrypted mempool like Shutter).
How do back-run rebates work?
After your trade, the pool is briefly out of balance — a harmless “back-run” can restore it and earn a small profit. Services like MEV Blocker and Flashbots Protect auction that opportunity to searchers and pay most of the proceeds (up to ~90%) back to you, instead of letting a bot keep it.
Which MEV protection should I use?
For everyday Ethereum wallets, MEV Blocker and Flashbots Protect are the top two private RPCs (rotate between them for big trades). MetaMask users can just toggle on Smart Transactions. For swaps specifically, 1inch Fusion or CoW Swap protect you by design.